The electric power steering apparatus is a machine that operates the motor for generating the steering assist force according to a steering torque applied by a driver. In recent years, the demand for electric power steering apparatuses for large-size automobiles has increased rapidly. Such a large-size automobile requires an increased steering assist force so that a larger amount of electric power must be supplied to the motor. In some cases, however, a battery alone is unable to fully meet such a large power demand. In this connection, an arrangement has been proposed wherein an auxiliary power supply is provided in addition to the battery such that the normal power demand is met by the battery alone but when a larger amount of electric power is required, the electric power is supplied by both the battery and the auxiliary power supply connected in series (see, for example, Japanese Unexamined Patent Publication No. 2005-287222 (FIG. 1)).
If the battery fails, the electric power steering apparatus stops working, resulting in a manual operation of the steering apparatus. In order to offer enhanced reliability, therefore, it has also been proposed to equip the electric power steering apparatus with an emergency power source for supplying the electric power in place of the battery such that the steering assist force may not be suddenly lost at the time of battery failure (see, for example, Japanese Unexamined Patent Publication No. 2006-213273 (FIG. 1) and Japanese Unexamined Patent Publication No. 2003-320942 (FIG. 2 to FIG. 6)).
However, if the electric power steering apparatus is provided with the auxiliary power supply for meeting the high power demand and further with the emergency power source providing for the battery failure, the apparatus has the larger number of power sources, so that components for controlling the charge/discharge of these power sources are also increased. This results in complicated circuit configurations and thence, in increased manufacture costs.